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Acquisitions and Dispositions - Additional Information (Detail) - USD ($)
$ in Thousands
9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Business Acquisition [Line Items]    
Purchase of non-controlling interests $ 60  
Proceeds from sale of property and equipment 47,000  
Capital expenditures, including exploration expense 40,057 $ 11,701
Mineral Acreage [Member]    
Business Acquisition [Line Items]    
Capital expenditures, including exploration expense 596  
Partnership And Trust [Member]    
Business Acquisition [Line Items]    
Purchase of non-controlling interests $ 60 $ 187